This invention is based on a process for working up the reaction gas comprising chlorine, hydrogen chloride, water vapour and residual oxygen produced in a reactor during the oxidation of hydrogen chloride to chlorine and water.
The conversion of HCl to chlorine has already been performed industrially by a range of processes. In addition to electrolysis of hydrochloric acid, there are two other types of process for the non-electrochemical conversion of HCl into chlorine. These are the wet chemical processes and the gas phase reactions. Wet chemical processes include the Kel-Chlor process [1] and Degussa""s H2O2 process [2]. The most significant gas phase processes are the following:
the Shell process [3]
the MTC process [4] and
the USC process [5].
These processes operate with a fluidised bed having copper chloride (USC, Shell) or chromium oxide (MTC) as catalyst on a porous support. The reaction gases, which consist of Cl2, HCl, O2, H2O, are worked up using various processes. In the USC process, the water of reaction is separated and discharged in one stage of a two stage reactor. Due to the elevated excess of oxygen required to convey fluidised bed material from one stage to the other, the chlorine formed in the second stage has a very high oxygen content and the inert gas cannot economically be removed from it by simple compression and liquefaction. For this reason, an absorber/stripper system is used, in which CCl4 is used as the adsorbent. However, the use of CCl4 on a large industrial scale is questionable on occupational health grounds and, in future, will even be prohibited. Another disadvantage is the complicated transport of large quantities of fluidised bed material between the two stages of the reactor, which must furthermore each be heated and cooled. The two stages are necessary in this case only for the selective separation of the water of reaction.
In contrast, the Shell process [3] operates in a single stage, such that the water of reaction must also be separated during working up. While [6] does describe a process which, by means of the careful operation of columns in the sub- and superazeotropic ranges, allows the complete recirculation of unreacted HCl gas into the reactor, the large quantities of inert gas which are necessary to operate the fluidised bed reactor mean that the chlorine content of the outgoing gas containing neither HCl nor H2O is so low that the conditions for subsequent chlorine liquefaction are relatively unfavourable. Consequently, here too absorption/desorption of the non-liquefied chlorine on CCl4 as the absorbent is used to separate the inert gases.
Even the complete separation of the water of reaction using sulphuric acid in a variant of the Shell Oil Company""s process [7] changes little with regard to the inert gas problem, such that a downstream absorber/stripper system with CCl4 as the absorbent had to be used here as well.
Finally, the MTC process [4] also operates with an excess of oxygen in the reactor, such that a downstream absorption/desorption stage with CCl4 as the absorbent must be used here as well. [8] furthermore describes performing the proposed chlorine purification using the so-called pressure swing adsorption process.
The object of the invention is to provide a complete process for working up the reaction gases during the oxidation of hydrochloric acid to isolate chlorine which operates without the absorption/desorption stages using extrinsic absorbents described above. The novel process is also intended fully to exploit the thermodynamic optimisation in the hydrochloric acid/chlorine system.
This object is achieved by the process stated in the main claim. Further developments and preferred embodiments are described in the subordinate claims.
The principal advantage of the process is that, with the exception of small quantities of sulphuric acid to dry the product gas, it is possible to dispense with the use of extrinsic media during working up of the product gases. Furthermore, the thermodynamic possibilities of the hydrochloric acid/liquid chlorine system are fully exploited in the individual stages of the process. In this manner, it is possible to dispense with problematic solvents, such as for example CCl4, in order to return to the process quantities of chlorine which have not been worked up. By avoiding the use of extrinsic water to eliminate any HCl present in the product gas, no unwanted dilute acid is produced.
The complete gas purification system downstream from the reactor having the stages:
injection condenser, phase separation apparatus, drying, recuperative liquefaction/distillation of chlorine, purge gas purification with hydrochloric acid by means of an absorber/stripper system and recirculation of the useful gases using the feed oxygen as carrier gas
constitutes a very energy efficient and well arranged process.